Fabrication of freestanding micro hollow tubes by template-free localized electrochemical deposition

ABSTRACT

The present invention provides a method of fabricating a micro hollow tube, more specifically, a method of fabricating a micro hollow tube by template-free localized electrochemical deposition, in which the micro hollow tube is fabricated by the accurate control of the distribution of the electric field strength during deposition with precise interplay of the applied voltage and the distance between the microelectrode and the grown structure.

This is a continuation of International Application PCT/KR2007/000325,with an international filing date of Jan. 19, 2007, currently pending,which is hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a method of fabricating a micro hollowtube. More specifically, the invention relates to a method offabricating a micro hollow tube by template-free localizedelectrochemical deposition, in which the micro hollow tube is fabricatedby the accurate control of the distribution of the electric fieldstrength during deposition with precise interplay of the applied voltageand the distance between the microelectrode and the grown structure.

BACKGROUND ART

The fabrication of micro-devices is a fundamental issue in moderntechnology. Diverse techniques have been developed to fabricatemicrostructures consisting of semiconductors, metals and polymers.Especially, freestanding three-dimensional (3D) hollow tubes areparticularly promising for broad applications in diverse areas such asoptics, electronics, medical technology and microelectromechanics.

Such structures are typically fabricated by conventional lithographicprocess, LIGA process (Marc J. Madau, Fundamentals of Microfabrication:The Science of miniaturization (CRC press, 1997)), track-etchmethod(Martin C R, Van Dyke L S, Cai Z, Liang W, J. Am. Chem. Soc. 112,8976 (1990)) and laser-assisted chemical vapor deposition (LCVD)(Lehmann O, Stuke M, Science 270, 1644 (1995)).

DISCLOSURE Technical Problem

So far, the most useful technique for producing 3D structures was theLIGA process using synchrotron x-rays—that combines lithography withelectrochemical metal deposition. Although quite successful, thisprocess is affected by some significant problems: it implies multiplefabrication steps, long fabrication times, high cost due to the use ofsophisticated masks or moulds; furthermore, the electroplating solutioncannot easily fill high aspect ratio trenches encountered during theprocess. In general, LIGA finds it difficult to produce complex 3Dstructures.

Technical Solution

Therefore, the present invention has been made in view of the aboveproblems, and it is an object of the present invention to provide a lowcost, fast and simple method for fabricating a micro-tube with the highaspect-ratio and uniform property.

To accomplish the above object, according to one aspect of the presentinvention, there is provided a novel method of fabricating a microhollow tube by template-free localized electrochemical deposition, themethod comprising the steps of: (a) placing a microelectrode (anode)very close to a substrate (cathode) immersed in a plating bath; and (b)applying a voltage greater than a critical voltage to the microelectrodeand the substrate via a electrochemical medium, and thereby to form amicro hollow tube structure on the substrate, wherein the criticalvoltage is defined as the applied voltage when a maximum electric fieldposition moves from the center of the end of the micro hollow tubestructure into the edge of the end of the micro hollow tube structurejust below the rim of the tip of the microelectrode.

Preferably, the method of fabricating a micro hollow tube furthercomprises (c) moving up the microelectrode from the formed micro hollowtube structure, with a contact growth mode being kept during deposition.

Preferably, a position of the microelectrode relative to the substrateor the micro hollow tube structure is directly observed by using animage collecting apparatus.

Preferably, the image collecting apparatus is a microradiographicapparatus with coherent X-rays in real time.

Preferably, the microradiographic apparatus comprises a X-ray beamsource, a sample stage, and an image detecting means.

Preferably, the movement of the microelectrode is performed with threestepping motors in sub-microns.

Preferably, the micro hollow tube comprises at least one selected fromthe group consisting of a metal and a metal alloy.

Advantageous Effects

In conclusion, we demonstrated that a careful manipulation of theelectric field strength distribution and in general of the growthparameters enables LECD to fabricate well-defined metallic micro hollowtubes. These results were made possible by a careful control of theinterplay between migration and diffusion, in turn determined by thefield strength.

DESCRIPTION OF DRAWINGS

The above and other objects, features and advantages of the presentinvention will be apparent from the following detailed description ofthe preferred embodiments of the invention in conjunction with theaccompanying drawings, in which:

FIG. 1 shows FE-SEM images of a 3D copper wire fabricated with anapplied voltage of 4.5 V and with an electroplating solution ofCuSO₄.H₂O (250 g/L), H₂SO₄ (75 g/L). In (a) we see an overall picture ofthe wire revealing two different growth regimes, illustrated in detailin (b) and (c). Specifically, (c) is the dense growth obtained for L≈40μm and (b) the porous one produced when L is reduced to a fewmicrometers. The top view in the inset of FIG. 1 b reveals ahollow-shaped feature with diameter not far from the 50 μm value of themicroelectrode (dashed circle);

FIG. 2 is the distribution map of the electric field strength fordifferent values of the applied voltage V and of the distance L. (a)V=4.5V and L=40 μm; (b) V=4.5V and L=5 μm; (c) V=10.0V and L=5 μm;

FIG. 3 is LECD growth of well-defined hollow tube. Top: FE-SEM imagesand (inset) tomographic slices of the grown structures; Bottom:real-time images of the LECD process by coherent x-ray microradiography.The images show: (a) a dense wire obtained at 4.5V in the no-contactgrowth mode; (b) a porous wire obtained at 4.5V in the contact growthmode; (c) a well-defined hollow tube obtained at 10.0V, again in thecontact growth mode by the method of the present invention. The contrastdifference in (c) between the inner (black arrow) and outer (whitearrow) regions in the radiographic image reveals the formation of ahollow tube.

MODE FOR INVENTION

The preferred embodiments of the invention will be hereafter describedin detail, with reference to the accompanying drawings.

We developed a novel approach based on localized electrochemicaldeposition (LECD) with significant advantages with respect to LIGA forthe fabrication of metallic micro hollow tubes. Specifically, it is asimple, inexpensive, and damage free method.

The LECD approach is based on electrochemical deposition: themicroelectrode (anode) is placed very close to the conducting substrate(cathode) immersed in the plating bath. As the voltage is applied andthe microelectrode is moved up, a metallic microstructure is fabricatedthat protrudes towards the microelectrode. The process is thusparticularly suitable for producing high-aspect-ratio metallicstructures with a variety of features. This simple approach can beapplied to different materials such as metals, metal alloys, conductingpolymers and semiconductors to fabricate objects in the micrometer,sub-micrometer, and nanometer scale.

We conducted experiments at room temperature using 1.05M CuSO₄.H₂O, 0.8MH₂SO₄. The microelectrode with 50 μm in diameter was prepared by sealingPt wire (99.95%, Alfa Aesar) in a glass tube and then by polishing thesurface. Platinum coated silicon wafers were used as cathodes. Themicroelectrode position was accurately controlled by three steppingmotors. The experiments were performed at the “7B2 X-ray Microscopy”beamline of the Pohang Light Source (PLS), Korea. Additional tests suchas field emission scanning electron microscopy (FE-SEM, JEOL JSM6330F)were also used to study the microscopic characteristics of the grownstructures. The microradiographic monitoring of the LECD process wasimplemented in situ in a specially designed miniature electrochemicalcell machined from a Teflon block and sealed by Kapton films that werex-ray transparent and stable for most chemical reactions. The distancebetween the two cell windows was optimized to ≈5 mm to avoid unnecessaryx-ray absorption by the plating electrolyte. For the micro-tomography,the grown structure was mounted on a translation/rotation stage withprecise positioning (250 nm/0.002 μm) and one thousand projectionradiographs were taken while rotating the sample between 0° and 180° Theslice images of the grown structure were then reconstructed by using aself-developed reconstruction algorithm.

FIG. 1 shows FE-SEM (field-emission scanning electron microscope) imagesof a 3D copper wire fabricated by the LECD process with an appliedvoltage of 4.5 V. FIG. 1( a) shows that the wire so produced reflectstwo growth regimes: the upper part [shown in detail in FIG. 1( b)]corresponds to a regime yielding a porous microstructure, whereas theother regime results in a dense uniform microstructure [FIG. 1( c)]. Thedense uniform growth was obtained with a relatively large distancebetween the microelectrode and the growing structure, L=40 μm(no-contact growth mode). The dense uniform growth abruptly changed to aporous growth when L was reduced to a few micrometers, (contact growthmode).

In order to understand this change in the growth characteristics, wemust consider the mass transport mechanisms of metal ions. Diffusion ofmetal ions from the bulk solution to the cathode dominates conventionalelectrochemical deposition; in LECD, however, we must take into accountthe migration of metal ions that is driven by strong localized electricfields. The distance L determines the interplay between diffusion andmigration. Specifically, diffusion prevails at large L-values whereasmigration increasingly dominates as L decreases. When L reaches thecritical value at which migration replaces diffusion as the dominatingfactor, the deposition rate rapidly increases because of the strongelectric fields, changing the grown structure from dense to porous asseen in FIG. 1. The top view shown in the inset of FIG. 1( b) reveals ahollow feature within the porous wire; the diameter of this feature isnot far from that of the microelectrode (dashed line).

One of the factors that affect the growth characteristics is theelectric field strength distribution near the grown feature. We modeledthis distribution and the results are illustrated in FIG. 2. For a lowapplied voltage of 4.5V and a large L-distance of 40 μm, the electricfield strength exhibits a maximum value at the center of the grownfeature [FIG. 2( a)]. We expect, therefore, the formation of a wire witha cone on top. As L decreases to 5 μm—a value much lower than thecritical level [FIG. 2( b)]—the maximum field position moves to the edgeof the grown structure just below the rim of the microelectrode. Thus,the formation of the porous region with the hollow feature of FIG. 1( b)can be explained by the electric field edge enhancement at themicroelectrode rim that induces a high migration rate below the rim.

As the applied voltage increases from 4.5 to 10V for L=5 μm, theelectric field strength sharply increases at the microelectrode rim,enhancing the field strength difference with respect to themicroelectrode core—see FIGS. 2( b) and 2(c). Consequently, theelectrochemical deposition is also enhanced below the rim. Theconsistent results of the field simulation and of the actual growth thussuggest that it is possible to change the copper grown structure from adense wire to a well-defined hollow shape simply by controlling theelectric field distribution near the microelectrode.

These findings lead us to suitable strategy to fabricate well-definedhollow tubes. The strategy is based on the control of the electric fieldstrength near the grown structure as suggested by FIG. 2: as the appliedvoltage is increased in the contact growth mode (the migration dominantregime), the enhancement of the growth below the microelectrode rimeventually produces a tube rather than a wire.

FIG. 3 is the results of this approach for the production of coppergrown structures on Pt substrates. The top of FIG. 3 shows FE-SEM imageswhile the bottom demonstrates microradiographic images obtained byreal-time coherent x-ray imaging. A dense wire is produced at 4.5V bythe no-contact growth mode [FIG. 3( a)]. The tomographic slicereconstruction in the inset of FIG. 3( a) shows that the wire is notonly dense but also uniform. The cone shape on top of the wire is theresult of the field-induced local migration discussed above—see FIG. 2(a).

On the other hand a porous structure is obtained at 4.5V in the contactgrowth mode as illustrated in FIG. 3( b) but a dense rim feature ispresent around the porous structure (white arrow), as confirmed by thex-ray tomographic slice in the inset of FIG. 3( b, top). As the appliedvoltage is increased to 10V, the grown structure changes to awell-defined hollow tube with a very uniform wall thickness (≈5 μm)—asshown by the FE-SEM image of FIG. 3( c, top) and by the correspondingtomographic slice in the inset. This is the limit result of themigration enhancement near the rim produced by a highly confined, strongelectric field. The coherent x-ray micro images of FIG. 3( c, bottom)illustrate this process in real time.

INDUSTRIAL APPLICABILITY

The practical cases discussed here are only a few examples of the broadvariety of metal structures that our novel LECD approach can produce byappropriate tuning of the growth parameters.

While the present invention has been described with reference to theparticular illustrative embodiments, it is not to be restricted by theembodiments but only by the appended claims. It is to be appreciatedthat those skilled in the art can change or modify the embodimentswithout departing from the scope and spirit of the present invention.

1. A method of fabricating a micro hollow tube by template-freelocalized electrochemical deposition, the method comprising the stepsof: (a) placing a microelectrode (anode) very close to a substrate(cathode) immersed in a plating bath; and (b) applying a voltage greaterthan a critical voltage to the microelectrode and the substrate via aelectrochemical medium, and thereby to form a micro hollow tubestructure on the substrate, wherein the critical voltage is defined asthe applied voltage when a maximum electric field position moves fromthe center of the end of the micro hollow tube structure into the edgeof the end of the micro hollow tube structure just below the rim of thetip of the microelectrode.
 2. The method of fabricating a micro hollowtube according to claim 1 further comprises (c) moving up themicroelectrode from the formed micro hollow tube structure, with acontact growth mode being kept during deposition.
 3. The method offabricating a micro hollow tube according to claim 1, wherein a positionof the microelectrode relative to the substrate or the micro hollow tubestructure is directly observed by using an image collecting apparatus.4. The method of fabricating a micro hollow tube according to claim 3,wherein the image collecting apparatus is a microradiographic apparatuswith coherent X-rays in real time.
 5. The method of fabricating a microhollow tube according to claim 4, wherein the microradiographicapparatus comprises a X-ray beam source, a sample stage, and an imagedetecting means.
 6. The method of fabricating a micro hollow tubeaccording to claim 2, wherein the movement of the microelectrode isperformed with three stepping motors in sub-microns.
 7. The method offabricating a micro hollow tube according to claim 1, wherein the microhollow tube comprises at least one selected from the group consisting ofa metal and a metal alloy.